1. Field of Invention
The present invention relates, in a general manner, to monitoring/control devices for lifting apparatus, and more particularly to safety devices such as load limiters, torque limiters, motion (lifting, rotation, distribution, translation) limiters or speed limiters, provided on lifting apparatus such as tower cranes.
More particularly, the invention pertains to the making secure of the processing of the signal of the sensors of these safety devices, usually embodied in the form of sensors of resistive analog type, supplied electrically with DC current.
2. Description of Related Art
Among monitoring systems of safety devices of lifting apparatus, one generally encounters systems employing so-called xe2x80x9cpassivexe2x80x9d analog sensors, that is to say ones which have to be supplied with power in order to operate, for example potentiometers, gauges or probes, these sensors being chosen on account of their low cost, their proven technology and their serviceability.
The use of monitoring systems of this kind, on lifting apparatus, requires that the acquisition pathway be made secure, this pathway conditioning and transmitting the analog signal delivered by a passive sensor such as this.
To make this acquisition pathway secure on lifting apparatus, the only known solution currently used is the redundancy of the circuits with comparison of the values acquired. Such a system is applied in particular on tower cranes constructed by the applicant company POTAIN.
The major drawbacks of this existing solution are its cost, its complexity of installation and its restricted availability since it leads to a doubling-up of the risks of malfunctioning.
The present invention aims to avoid these drawbacks, by providing a solution which, for identical security, is appreciably simpler and more economical, and reduces the risks of malfunctioning of the system.
Accordingly, the subject of the invention is essentially a process for making secure the processing of safety signals, more particularly of a signal delivered by a sensor of resistive analog type, supplied electrically with DC current, in a safety device on a lifting apparatus, the analog signal delivered by the sensor being converted into a digital signal in an analog/digital converter of given resolution, the process being characterized in that:
at a first instant, a digital signal corresponding to the analog signal delivered by the sensor is acquired and stored,
then the supply voltages of the sensor are inverted,
after this inversion, at a later instant, a new digital signal corresponding to the analog signal delivered by the sensor is acquired,
the two digital signals acquired are summed,
the sum of these two digital signals is compared with the resolution of the analog/digital converter, and
if this sum is equal or approximately equal to the resolution of the converter, a control signal for operating an actuator of the lifting apparatus, corresponding to a control pre-set, is enabled, or
if said sum is significantly different from the resolution of the converter, the control signal is not enabled.
Thus, the principle of the invention relies on the possibility of checking the operation of the pathway for acquiring the signal of a resistive analog sensor, simply by inverting the supply voltages of this sensor.
In the event of correct operation, the two digital signals, acquired respectively before and after this inversion, are xe2x80x9ccomplementaryxe2x80x9d, that is to say their sum is constant, and equal to the resolution of the analog/digital converter. Conversely, in the event of malfunctioning, the sum of the two relevant digital values is no longer equal to this resolution, thus allowing this malfunctioning to be readily detected.
According to a preferred mode of implementation of the process which is the subject of the invention, the analog signal, delivered by the sensor, is filtered and matched, so as to yield a digitizable analog signal which is the signal to be converted, before and after inversion of the supply voltages of the sensor.
Advantageously, in the case where the calculated sum of the two digital signals is significantly different from the resolution of the analog/digital converter, a signaling is emitted to indicate the state of malfunctioning.
The subject of the invention is also a device for the implementation of the process defined above.
This device is defined, essentially, as a device for making secure the processing of safety signals, more particularly of the signal delivered by a sensor of resistive analog type, supplied electrically with DC current, in a safety device on a lifting apparatus, the sensor being linked to an analog/digital converter of given resolution, which converts the analog signal delivered by the sensor into a digital signal transmitted to an arithmetic and logic processing unit which also receives a control pre-set, and which is able to deliver a control enable signal for operating a motion actuator for moving the lifting apparatus, the device being characterized in that it comprises, in combination:
control means for inverting the supply voltages of the sensor,
means for acquiring the digital signals corresponding to analog signals delivered by the sensor, respectively before and after inversion of its supply voltages,
means of summing the two digital signals acquired, and of comparing the sum of these two signals with the resolution of the analog/digital converter, and
means of enabling/disabling the control of the actuator, corresponding to the pre-set, according to the result of the comparison of the aforesaid sum with the resolution of the converter.
The sensor being in particular a potentiometer, the means of control of inversion carry out the inversion of the supply voltages present at the terminals of the potentiometer.
Preferably, an analog conditioning circuit, intended for filtering and matching the analog signal delivered by the sensor, such as a potentiometer, is interposed between this sensor and the analog/digital converter.
In an advantageous embodiment of the device of the invention, the means of acquiring the two digital signals, of summing these signals, of comparing the sum of these signals with the resolution of the analog/digital converter, and of enabling/disabling the control according to the result of the comparison, are incorporated into the arithmetic and logic processing unit, the latter possessing an output linked to the means of control of inversion of the supply voltages of the sensor, which means are embodied in the form of an inversion circuit.
Advantageously, the arithmetic and logic processing unit, incorporating (as indicated above) the means of acquiring the two digital signals, of summing these signals and of comparing their sum with the resolution of the converter, is embodied in the form of a microprocessor.
By virtue of the process and of the device which are the subject of the invention, it becomes possible to implement a single sensor, instead of two sensors (for redundancy of the circuits, according to the prior technique), whilst still obtaining identical security, or even improved security. The invention also leads to a simplification of the wiring of the relevant lifting apparatus, since it requires the wiring of just one sensor, instead of two, hence a saving. Moreover, the invention diminishes the causes and decreases the risks of malfunctioning, since it implements a single security circuit, instead of two circuits. Finally, the system proposed by this invention is almost 100% reliable, and self-monitored: if the security circuit is itself defective, it does not instruct the inversion of the supply voltage of the sensor, so that the complementarity of the two digital signals is no longer ensured, and hence the system goes faulty.
The invention will in any case be better understood with the aid of the description which follows, with reference to the appended diagrammatic drawing representing, by way of example, an embodiment of this device for making secure the processing of safety signals, on lifting apparatus: